The operator of the Fukushima Daiichi nuclear power station says 3 of the plant's 6 reactors were shaken on March 11th by tremors exceeding forces they were designed to withstand.

The Tokyo Electric Power Company, known as TEPCO, says reactor No.2 suffered the largest horizontal ground acceleration of 550 gals, which is 26 percent stronger than the reactor's design limit.

"gal, unit of acceleration, named in honour of the Italian physicist and astronomer Galileo Galilei (1564–1642) and used especially in measurements of gravity. One gal equals a change in rate of motion of one centimetre (0.3937 inch) per second per second."

http://www.youtube.com/watch?v=TLZdArVij5E

http://www.youtube.com/watch?v=T_N-wNFSGyQ

TEPCO says the readings were 548 gals at the No.5 reactor, about 21 percent higher than its design limit; and 507 gals at the No.3 reactor, topping the capacity by about 15 percent.

The power company says the strength of ground motions were close to or within the design parameters at the remaining 3 reactors, and at all 4 reactors of the nearby Fukushima Daini nuclear plant.